A full-track vehicle has a pair of tracks usually connected through a pair of differentials to a single drive engine. The differentials are interconnected by a steering shaft or similar element which is rotatable in one sense to advance one of the tracks faster than another, thereby causing the vehicle to turn to one side, and rotatable in the opposite sense to advance the other track faster than the one track so as to cause the vehicle to veer to the other side.
This steering shaft interconnecting the differentials is connected to one part of a three-part planetary-gear assembly which has another part that can be arrested by a brake and yet another part which is connectable through an intermediate planetary gear assembly to a steplessly adjustable drive. Typically this drive comprises a pump and motor arrangement having an input side connected to the drive engine and an output side connected to one part of the three-part intermediate planetary gear assembly.
In one known system the sun gear of the primary planetary gear assembly is connected with the output part of the intermediate planetary gear assembly. The output side of the steplessly variable drive of transmission connected between the engine and this intermediate planetary gear assembly therefore allows relatively fine steering control to be obtained. In such a system the ring gear of the intermediate planetary gear assembly is both connected to a brake and through a coupling or a bevel-gear assembly to the drive engine so as to permit it to be arrested or driven in the opposite direction.
Such an arrangement has the disadvantage that it does not provide uniform steering deflection. Thus, for instance, relatively minor steering deflection is obtained by arresting the ring gear of the intermediate gear assembly with the brake and driving the primary gear assembly through this intermediate assembly with the steplessly variable transmission. A sharper turn is effected by opening the brake and reverse driving this ring gear with the drive engine. Typically the switchover from the instant in which the ring gear is arrested and that in which it is driven in the opposite direction causes the vehicle to veer sharply, therefore making steering somewhat difficult. It has been found almost impossible in this type of system to eliminate this jumpy steering.
A simple manner of overcoming this difficulty has been to provide very large and powerful steering drive which has an input side connected to the drive engine and an output side connected through gearing to the steering shaft. This heavy-duty drive is steplessly variable so that it alone rotates the steering shaft in the sense necessary to turn the vehicle in either direction and therefore must all alone dispose of sufficient power to carry out any necessary steering operations. Such a system, although relatively sure and simple, has the disadvantage that it is necessary to provide an extremely large steering drive in order, for instance, to make a sharp turn while climbing a hill or during a similar operation.